Silicon wafer thinning is an important step in the manufacture of semiconductor devices and micro-electro mechanical systems (MEMS). Wafer thinning is vital because it aids in preventing heat build-up in the wafer during manufacture and use, and also makes the wafer easier to handle and less expensive to package.
Traditionally, the wafer thinning process has been performed by grinding and polishing operations commonly referred to as “backgrinding,” or by using solutions containing strong oxidizers such as nitric acid (HNO3) and/or hydrofluoric acid (HF). These two processes are also often combined, because the mechanical grinding operation induces a significant amount of stress in the silicon surface. This stress may be alleviated by chemical etching, which removes the stressed and damaged layer. The chemical reactions for this process generally proceed as follows:Si+2HNO3−>SiO2+2NO2 (silicon oxidation)4HF+SiO2−>SiF4+2H2O (dissolution of silicon dioxide)
From these reactions, it is apparent that the silicon removal mechanism is the formation of silicon dioxide by exposure to an oxidizing agent (HNO3), followed by the reaction of the silicon dioxide with fluorine to form silicon tetrafluoride (SiF4), which can be dissolved in an aqueous carrier or evolved as a gas. While numerous oxidizing agents have been experimented with, there appear to be few alternatives to fluorine for the reaction to effectively proceed.
One problem with this process is that it is difficult to control, due to the consumption of reactants and the evolution of nitrous oxides which dissolve into the etchant solution, thereby “poisoning” the bath by saturation, which will affect subsequent etches. The process also requires large volumes of expensive process chemicals, involves a great deal of hazardous waste, and is difficult to control to the extent required in order to deliver optimal etch uniformity.
Alternative silicon etchants include caustic solutions such as potassium hydroxide or sodium hydroxide, or fluorine plasma chemistries such as SiF6. The two primary classes of silicon etchants can thus be classed as either aqueous chemistries applied in the liquid state (e.g., HNO3, HF, or caustics), or fluorine plasmas. Each of these process categories has certain applications and limitations.
In the case of aqueous chemistries, limitations include the cost of the chemicals, the need for significant amounts of water for rinsing, the creation of large volumes of waste (chemical and rinse water), and the inability to deliver the etchant into small geometries, which are common in semiconductor devices and MEMS devices. In general, caustics are not favored in the semiconductor industry, due to commonly known problems associated with mobile ion contamination. This is especially true of elements such as Na and K.
In the case of plasma chemistries, the cost of the processing equipment and supporting hardware (e.g., vacuum pumps) can be quite high. Many plasma processes are designed to deliver an anisotropic etch profile. While this has many desirable features, it has a tendency to create very sharp corners on system geometries, which can lead to device breakdown. Many of these systems are also designed for single-wafer processing, which can prove detrimental, depending on throughput requirements. Additionally, plasmas are relatively expensive, dirty, and are not configured for the removal of several hundred microns of silicon as required in a wafer thinning operation.
Thus, there is a need for an improved method of thinning silicon wafers used in semiconductor devices and/or MEMS devices.